Post on 23-Feb-2016
description
Current Projects
Alan Fisher
APE Meeting2010-08-03
Ongoing Projects
Terahertz radiation from the LCLS electrons With Aaron Lindenberg of PULSE
Measuring short SPEAR bunches by cross correlation of synchrotron light with a Ti:sapphire laser
With Jeff Corbett (SSRL) and Aaron Lindenberg Imaging the LHC protons with synchrotron light
With several CERN collaborators Beam-loss monitoring for machine protection at LCLS Catching gas bursts near the LCLS gun Assorted LCLS measurements
Terahertz Radiation
Electrons pass through beryllium foil downstream of undulator 2-µm foil at 45° to beam, with diamond window below
THz radiation extracted from wake of compressed beam Femtosecond pulses with intense fields:
Peak electric field expected to be ~3 V/Å Laboratory sources for 1-10 THz are much weaker: < 0.01 V/Å
Peak magnetic fields of order 100 T Beam diagnostic: Single-shot characterization of fs bunches User experiments: Switching materials at the level of atoms,
optical manipulation and control of structural and electronic properties, measuring the speed of material transformations
Later: THz/X-ray pump/probe experiments in the NEH Requires a long THz transport line
Calculated Field and Spectrum at Focus
Calculation by Henrik Loos for 1 nC and 20 fs
bolometer
Pyro detector
Pyro cam.
THzAuto-correlation
Sample stage/pinhole xyz stage
flip mirror
iris
Motorized filter set
Laser
QWP
ZnTe
BS
EO samplingBalanced
Diodes/Andor
T2A
T,2A
T
2A
3T2A
RR
T
Alignmentlaser
2A
Delay stage
2A
BS2A
Half wave plate R
flip mirror
Layout of THz Optical Table
Laser specs 800-nm wavelength 20-fs pulses 68-MHz repetition rate 150 mW average power
First: THz characterization Energy per pulse
Bolometer and pyroelectric detector Focal-spot size
Pyroelectric camera moved through focus
Later: Diagnostics and samples Electro-optic sampling Michelson interferometer First samples in focused THz beam
e−
Putting the First Optics on the Table
Restricted, Parasitic, and Full-Time Use
Both electrons and x rays hit the foil Negligible loss for photons > 2 keV: parasitic use possible Can’t insert foil when users want soft x rays
Perhaps…bump electrons away from x rays at foil Electrons pass through foil, and then return to center of beampipe
before bending down to the dump Let x rays miss the foil by skirting its edge or by going through a hole Foil could be used at all times What bump amplitude can we make? Can we cut a hole or support a free edge in a Be foil?
Thickness limited to 5 µm by Radiation Physics
LHC Synchrotron-Light Monitors
Two applications: BSRT: Imaging telescope, for transverse beam profiles BSRA: Abort-gap monitor, to verify that the gap is empty
Particles passing through the abort kickers during their rise get a partial kick and might quench a superconducting magnet.
Two particle types: Protons and lead ions
Three light sources: Undulator radiation at injection (0.45 to 1.2 TeV) Dipole edge radiation at intermediate energy (1.2 to 3 TeV) Central dipole radiation at collision energy (3 to 7 TeV) Spectrum and focus change during ramp
Layout: Emission and Extraction
To RF cavities and IP4
To arc
Cryostat
Extracted light sent to an optical table below the
beamline
1.6 mrad
70 m
26 m 937 mm560 mm
420 mm
D3 U
10 mD4
194 mm
Optical Table
Alignment laser
Focustrombone
F1 = 4 m
PMT and 15% splitter for abort gap
Intermediate image
Table Coordinates [mm]
CamerasSlit
Calibration light and
target
F2 = 0.75 m
Beam
Table
Extraction mirror
Shielding
Telescope for Beam 1
Beam-1 Extraction mirror(covered to hunt for light
leak)
Door toRF cavities
Undulator and dipole
Beam 1 Beam 2
Optical Table
Beam 1 Beam 2
Light from undulator.No filters. Open slit.
LHC Beams at Injection (450 GeV)
Horizontal1.3 mm
1.2 mm
Vertical0.9 mm
1.7 mm
Beam 1 Beam 2
Horizontal0.68 mm
0.70 mm
Vertical0.56 mm
1.05 mm
LHC Beams at 3.5 TeV
Light from D3 dipole.Blue filter. Narrow slit.
Calibrating the Abort-Gap Monitor
Inject a “pilot” bunch Charge measured by bunch-
charge and DC-current electronics Attenuate light by ratio
bunch charge / quench threshold
Move BSRA gate to include the pilot bunch
Find PMT counts per proton (adjusted for attenuation) as a function of PMT voltage and beam energy
Turn RF off (coast) for 5 minutes to observe a small, nearly uniform fill of the gap
Useful to test gap cleaning…
Last bunch in fill First bunch in fill
Abort gap
Time [100-ns bins]
After coasting briefly, bunch spreads out
Pilot bunch
Test of Abort-Gap Cleaning, December 2009Abort gap (3 µs)
Position in fill pattern (100-ns bins)
Tim
e (s
)
Charge drifting into gap
Cleaning excites beam at transverse tune. Applied to a 1-µs region: immediate effect.
Excitation had ringing on the trailing edge (improved in January)
Beam dumped
RF off: coasting bunch in bucket just after gap
Gas Bursts in the LCLS Injector
Gas burst near gun can lower photocathode’s QE Archiver records pressures at 1 Hz: too low to track the source Beam-synchronous multichannel digitizer (120 Hz) records all
pressures in a long circular buffer. Buffer saved automatically after each burst
One culprit found: “Guardian” software that halted beam by shutting off low-level RF drive to first linac section (L0A)
Pressure burst when Guardian is reset and L0A restarts
Gas Burst from L0A Restart
Pressure RF Amplitude